In many commercial and industrial applications, refrigeration or air-conditioning systems are continuously operated irrespective of seasonal changes. In, for example, laboratories or computer installations which require maintenance of constant temperature conditions, the heat load on the refrigeration system is substantially the same in summer or winter. In air-cooled refrigeration systems, it is customary to locate the condenser outdoors thus subjecting the condenser to a wide variety of climatic conditions, especially in geographical locations which experience the full range of seasonal changes.
To maintain the cooling capacity of a refrigeration system, it is essential that an adequate pressure differential be maintained across the thermal expansion valve to maintain an adequate mass flow of refrigerant through the system. As is well known, the condensing pressure within an air-cooled condenser is a function of outdoor ambient temperature. As outdoor ambient temperature drops, pressure within the condenser also drops while the condensing rate increases resulting in a decrease in the mass flow of refrigerant through the expansion valve due to the decrease in pressure drop across the expansion valve.
The term "condensing pressure" is used synonymously with head pressure or high-side pressure. The "high side" of a refrigeration system is defined as that part of the system between the discharge side of the compressor and the inlet to the expansion valve. That part of the system between the outlet of the expansion valve and the suction side of the compressor is conventionally referred to as the low side.
Operating a refrigeration system at too low a high-side pressure can result in serious problems. For example, oil may be trapped in the condenser, the evaporator can be starved due to the insufficient mass flow of refrigerant through the expansion valve, and the suction gas superheat can reach a point where it cannot dissipate the heat of the compressor, thus causing the compressor to run hot.
A number of solutions to the problem of maintaining adequate high-side pressure in air-cooled condensors have been proposed. One such means is that of controlling the refrigerant flow in response to changes in condensing pressure, sometimes referred to as a "flooded condenser" system. However, this method is quite costly and presents a number of operational difficulties, in that a flooded condenser system requires nearly twice the refrigerant charge as compared to a conventional system. Moreover, the greater the refrigerant charge, the greater the problem of refrigerant migration becomes which can, for example, cause lock-outs of the oil pressure switch and cause damage to the compressor due to "liquid slugging". In addition to the increased size of equipment such as the receiver, an elaborate and costly pressure control system must be provided.
Another means of controlling high-side pressure is termed "air side control", wherein dampers are used to regulate the air flow across the condenser coils. The dampers are operated either in response to condenser pressure or temperature or can be operated by a piston device driven by the discharge pressure of the system. This type of control also suffers from several limitations and drawbacks. For example, the condenser fans must be of the non-overloading type, for when the dampers are closed, the increased static pressure will cause an increase in the fan motor current. Although this problem could be eliminated by installing a face and by-pass damper arrangement, the same is expensive and occupies too much of the interior space in the condenser. Even so, the dampers are prone to icing under wintry weather conditions and, as a result, often jam in the open or closed position, either of which could cause serious damage to the system.
It has also been proposed to control high-side pressure via the condenser fan itself. In such systems, a low range reverse-acting high pressure switch is used to sense high-side pressure and is connected in the fan circuit to stop the fan when the head pressure falls below a predetermined point and to start the fan when the head pressure rises to a predetermined point. In a variation of this control means, a modulated speed fan rather than an on-off fan is used the fan speed decreasing with decreasing high-side pressure and vice versa. However, there are two major drawbacks to the use of either on-off or variable speed fans. In the first instance, if such controlled fans are used with a horizontal condenser coil, wind velocity can prevent pressure from building up on the high side, thus causing the system to operate at a low head pressure and, if the load in the system under such conditions is normal, the superheat at the compressor could increase to a point causing burn-out of the compressor. In the second instance the said fan control systems are generally limited in their ability to maintain adequate high-side pressure whenever the outdoor ambient temperature falls below about 40.degree.F.